As device dimensions shrink, it becomes difficult to fill trenches and vias with copper in a damascene copper interconnect process. For example, copper can be used to fill a high aspect ratio trench/via through a migration process. That is, it is known that copper migrates at a much lower temperature than its melting point when the copper film has a thickness less than a few nanometers. This copper migration tends to accumulate at the bottom of the trench and/or via due to the so-called capillary phenomenon. More specifically, by utilizing this phenomenon, small trenches and/or vias are filled by: (1) deposition of ultra thin copper films (e.g., as thin as less than a few nanometers), and (2) heating the wafers at temperatures as low as 200° C. to 400° C. At these temperatures, the ultra thin copper films migrate to the bottom of the trenches and/or vias to partially fill the features from the bottom side. Since the actual aspect ratio which needs to be filled with a subsequent plating of copper becomes low, it becomes easier to fill such high aspect ratio features with copper plating.
However, due to the migration of the copper atoms at the interface of the underlying materials, e.g., diffusion barrier metals such as TaN and Ta, formed on sidewalls, the underlying materials become exposed, which makes subsequent electroplating of copper difficult. This is because it is difficult for the copper to adhere to the underlying exposed materials, e.g., TaN and Ta. Also, it is known that the migration of copper causes agglomeration of copper at the sidewalls of the trench/via which, in turn, forms isolated islands. These isolated islands, though, do not migrate to the bottom of the trench/via features because of the lost capillary phenomenon. It is also known that these agglomerated copper islands make it difficult for filling of the trenches and/or vias with electroplated copper. In other words, the low temperature migration of copper atoms which takes place when the copper film is ultra thin causes both agglomeration and migration of copper at the same time, which results in processing concerns.
More specifically, in conventional processes, after trench/via pattern definition, a liner of copper seed is formed along the sidewall of the trench/via before electroplating of copper. In the conventional process, when copper is heated for reflow, copper islands are formed due to the dewetting of the liner surface (i.e., disconnected flow of copper down to the bottom of the trench/via features). A resultant void is thus formed after the copper electroplating. Also, from the dewetted liner surface, no electroplating takes place, or early pinch off of the trench entrance occurs due to the copper island formation. In this latter situation, voids will remain in the trench/via features.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.